Transparent LAS glass ceramics produced with alternative environmentally acceptable fining agents

ABSTRACT

The transparent glass ceramic with low thermal expansion is produced from a glass that is fined with a combination of environmentally friendly fining agents, specifically a combination of SnO 2  with at least one of Sb 2 O 3 , SO 4   2− , Br −  and Cl − . The glass is free of arsenic and is preferably made from a mixture of SiO 2 , Al 2 O 3 , Li 2 O, TiO 2 , ZrO 2  and ZnO. A mirror support for astronomy and a prism for LCD lithography are advantageously made from this glass ceramic.

CROSS-REFERENCE

The invention described and claimed herein below is also described inGerman Patent Application DE 10 2010 002 188.1, filed on Feb. 22, 2010in Germany. The aforesaid German Patent Application, whose subjectmatter is incorporated herein by explicit reference thereto, providesthe basis for a claim of priority of invention for the invention claimedherein below under 35 U.S.C. 119 (a)-(d).

BACKGROUND OF THE INVENTION

1. The Field of the Invention

The present invention relates to transparent glass ceramics having lowthermal expansion, also called “zero expansion”, containing onlyenvironmentally acceptable fining agents, or being fined only withenvironmentally acceptable fining agents, respectively, as well as tothe uses of the glass ceramics according to the present invention.

2. The Related Art

Among glass ceramics having low thermal expansion,lithium-aluminium-silicon oxide glass ceramics, which are also known inthe art as LAS glass ceramics, are so-called “zero expansion materials”.A well-known type of this sort of glass ceramic material, known asZERODUR®, is commercially available in three different thermal expansionclasses of 0 to 2 and has different coefficients of linear thermalexpansion a in a temperature range of 0° to 50° C. as shown in Table Iherein below.

TABLE I Linear Thermal Expansion Coefficients of Different ClassesCoefficients of linear Expansion class thermal expansion α 0 0 ± 0.02 ·10⁻⁶/K 1 0 ± 0.05 · 10⁻⁶/K 2 0 ± 0.10 · 10⁻⁶/K

With respect to melting of glass the term “fining” means the removal ofgas bubbles from the melt. To achieve the highest quality in terms ofremoval of foreign gas and bubbles a thorough mixing and degasifying ofthe molten mixture is necessary. The behavior of gases and bubbles,respectively, in the glass melt as well as their removal are describedfor example in “Glastechnische Fabrikationsfehler”, edited by H.Jebsen-Marwedel and R. Brückner, third edition, 1980, Springer Verlag,on pages 195 ff.

Chemical fining methods are most often used. Their principle ofoperation is that compounds which undergo decomposition and releasegases, compounds which are volatile at elevated temperatures orcompounds which release gases in an equilibrium reaction at highertemperatures are added to the melt.

Sodium sulfate, for example, which is used for fining soda-lime glass,belongs to a first group of these compounds used for chemical fining. Inthis case, the release of SO₂ and O₂ takes place in a temperature rangeof 1300° C. to 1450° C. with a maximum at 1380° C. This temperaturerange approximately corresponds to the fining range of such glasses.

Compounds, which are volatile at high temperatures due to their vaporpressure and thus act as a gas, such as halides, sodium chloride ordifferent fluorides, belong to a second group. Thus for example a seriesof boron silicate glasses are fined with sodium chloride. The finingagents of this second group are designated “volatilization finingagents”.

A last group of fining agents comprises the so-called redox finingagents, such as arsenic oxide and antimony oxide. In practice, they areused most often. In the respective fining methods covalent ions, whichcan be present in at least two inter-related oxidation states in atemperature-dependent equilibrium with each other so that a gas, usuallyoxygen, is released at high temperatures, are used as redox finingagents.

For arsenic oxide as an example, the redox equilibrium that occurs inthe melt can be represented by the following equation (I):As₂O₅

As₂O₃+O₂↑  (I),

The equilibrium constant K for equation (I) is given by equation (II):

$\begin{matrix}{{K(T)} = {\frac{{aAs}_{2}{O_{3} \cdot {z\left( O_{2} \right)}}}{{aAs}_{2}O_{5}}.}} & ({II})\end{matrix}$

In this equation aAs₂O₃ and aAs₂O₅ represent the respective activitiesof the arsenic trioxide and arsenic pentoxide and z (O₂) represent thefugacity (of the oxygen). The equilibrium constant K strongly depends onthe temperature and a defined oxygen fugacity z (O₂) can be adjustedwith the temperature and the activity of the oxidic arsenic compounds.

A disadvantage of many redox fining agents is that they are harmful tothe environment; at least they are not environmentally acceptable.Particularly this applies to arsenic oxide. However this particularfining agent has a property which is useful: The release of the fininggas O₂ has two maxima at about 1250° C. and at about 1600° C. Thesemaxima correspond to the temperature range of melting, on the one hand,and the temperature range of fining, on the other hand. This sort offining behavior is desirable, but arsenic trioxide is strongly toxic andundoubtedly classified as carcinogenic. Anionic arsenic is alreadypresent as arsenite and arsenate in high concentrations in the groundwater of many countries. This is the reason that As₂O₃ is to beforbidden in several countries in the next years.

DE 1 596 860 discloses a method for the production of transparent glassceramics having low thermal expansion. As a fining agent 0.3 to 0.5% byweight of As₂O₃ is used. Environmentally acceptable fining agents arenot discussed.

U.S. Pat. No. 4,851,372 describes transparent glass ceramics which arefined with 0 to 1.5% by weight of As₂O₃ or Sb₂O₃. Also in this case,there is no discussion about environmentally acceptable fining agents.

EP 1 864 952 A1 describes aluminium silicate and lithium aluminiumsilicate glass ceramics containing amounts of SnO₂ and/or CeO₂ of 0.01to 5.0 percent by mass each, based on the oxides. However, the ceriumoxide affects the transmissions of the glass ceramics.

US 2007/0281849 A1 describes glass ceramics containing amounts of SnO₂and/or CeO₂ of 0.01 to 5.0 percent by mass, based on the oxides. Thepresence of cerium oxide is a disadvantage, because it may result in anundesired coloration.

DE 199 39 771 A1 describes a fining method for glass melts during whicha fining gas is produced in the glass melt by a fining agent. As finingagents redox compounds, in particular redox oxides such as SnO₂, CeO₂,Fe₂O₃, ZnO, TiO₂, V₂O₅, MoO₃, WO₃, Bi₂O₅, PrO₂, Sm₂O₃, Sb₂O₅, Eu₂O₃,TbO₂ and/or Yb₂O₃, can be used. In addition, metal oxides, such as ZnO,As₂O₃, Sb₂O₃, Bi₂O₃ and/or SnO, are added, which release oxygen andchange into the metallic state, during the fining operation. Many ofthese fining agents and their combinations, such as CeO₂ and Fe₂O₃,change the transmission properties of the glass ceramic in adisadvantageous manner.

SUMMARY OF THE INVENTION

It is the object of the present invention to provide transparent glassceramics having low thermal expansion which contain alternativeenvironmentally acceptable fining agents and preferably avoid thedisadvantages of the above-described environmentally unacceptable finingagents.

The object of the present invention is attained by the subject matter ofthe patent claims appended herein below.

The object is in particularly attained by a transparent glass ceramichaving a low thermal expansion, comprising the following composition, %by weight, based on oxide content:

SiO₂ 35 to 70 Al₂O₃ 17 to 35 Li₂O 2 to 6 TiO₂ 0 to 6 ZrO₂ 0 to 6 TiO₂ +ZrO₂ 0.5 to 9 ZnO 0.5 to 5,wherein the glass ceramic is produced by fining with SnO₂ and at leastone further fining agent.

The method of producing these glass ceramics comprises adding to acrystallizable glass a mixture of fining agents that permits productionof a glass ceramic having low thermal expansion without using arsenicoxide. Preferred mixtures of fining agents are mixtures of SnO₂ with thefurther fining agents Sb₂O₃, SO₄ ²⁻, Br⁻ and/or Cl⁻. In order to obtainglass ceramics with good transparencies, the fining agents should beused in the correct proportions, i.e. in a preferred molar ratio of SnO₂to further fining agents of 1:2 to 2:1.

The respective fining method of the invention is a so-called mixedfining. In production the fining using SnO₂ as a fining agent results ina chemical equilibrium represented by the following equation (III):2SnO₂

2SnO+O₂  (III),

Thus the glass ceramic according to the present invention comprises SnO,which has been formed from SnO₂, and at least one further component,which has been formed from a fining agent preferably selected from thegroup consisting of Sb₂O₃, SO₄ ²⁻, Br⁻ and Cl⁻.

Preferably the respective components, which result from the finingagents SnO₂ and Sb₂O₃ or SO₄ ²⁻ or Br⁻ or Cl⁻, are contained in theglass ceramic according to the present invention. When SO₄ ²⁻, Br⁻ orCl⁻ act as fining agents, they are added to the starting melt in theform of their alkali or alkaline earth salts.

According to the present invention combinations of fining agents areprovided which can provide similar fining conditions as As₂O₃ which hasalready been described above as an environmentally unacceptable finingagent due to its toxic and carcinogenic properties. In this case, thefining agents satisfy the further condition that no significant changesin a proven and tested melting process are necessary. A nearly similarfining effect can be achieved in comparison to the fining agent arsenictrioxide without the need for a change in process parameters. Thissignificantly reduces costs and thus allows an economically advantageousproduction of the ceramics according to the present invention.

Sb₂O₃ as a fining agent has a high O₂ release peak at about 1150° C.which overlaps the O₂ release peak of As₂O₃ at 1250° C. The sulfatereleases SO₂ and O₂ at about 1300° C. Thus, the combination of bothfining agents is suitable for replacing the toxic As₂O₃. Br⁻ and Cl⁻ arereleased as HX (X=Cl, Br) already at lower temperatures. This results inthe release of less fining gas than Sb₂O₃ or sulfate at highertemperatures (>1200° C.) in the case of these latter fining agents.Therefore these latter fining agents are used in a supportive manner.

In the selection of suitable combinations of fining agents the fact thatnone of the fining agents used should cause coloration of the glassceramic product must be considered. Therefore these glass ceramics arepreferably not fined with CeO₂ so that the glass ceramics areessentially free of CeO₂.

According to the present invention the glass ceramic contains at leastone component, which has been formed from a fining agent, in an amountof 0.001 to 2% by mol each, based on the total amount of glass ceramicpresent.

When it is said that the glass ceramics are “free of”, “do not contain”,or are “essentially free of” a certain component or a certain componentis not used in the preparation of the glass ceramic, this means that therespective component can only be present as an impurity in very smallamounts. These very small amounts are typically below 100 ppm,preferably below 50 ppm.

Preferably the present glass ceramics are free of iron oxides becauseiron can affect the transmissions of the glass ceramics of the presentinvention. The glass ceramics are also preferably free of vanadiumoxides.

The glass ceramic according to the present invention preferably containsa component resulting from a fining agent in an amount of 0.001 to 2% bymol.

The glass ceramic according to the present invention preferably containsa component resulting from the fining agent SnO₂, in particular SnO, ina proportion of 0.001 to 2% by mol.

In the present invention the term “zero expansion materials” means glassceramics with a coefficient of linear thermal expansion that is zero ornear zero in a defined temperature range.

The glass ceramics according to the present invention have a thermalexpansion of expansion class 2, preferably of expansion class 1, andmore preferably of expansion class 0. A special material, ZERODUR®K20,which for example is produced from ZERODUR® by a further step ofadditional ceramization has a coefficient of linear thermal expansion,or expansion of length, of 1.5 ppm/K.

The glass ceramic according to the present invention preferablycomprises one or more of the following further components, in % byweight, based on oxide content:

B₂O₃ 0 to 6 Na₂O 0 to 2 K₂O 0 to 2 MgO 0 to 5 P₂O₃ 0 to 17 CaO 0 to 4BaO 0 to 5 SrO 0 to 5.

Preferably the glass ceramic according to the present invention does notcontain As₂O₃, CeO₂ and/or PbO. As₂O₃ and PbO are toxic and are a heavyburden for the environment. CeO₂ results in a very strong self-coloringof the glass ceramic which is not desirable for the glass ceramicsaccording to the present invention. This change in color may possibly befurther intensified by formation of a colored complex of TiO₂—CeO₂.

A further reason not to use of CeO₂ is that in the production process ofthe glass ceramic oxygen is released already between 950° C. and 1050°C. CeO₂ has a release peak at about 1000° C. which is too low forreplacing As (first release peak at 1250° C.). But the finingtemperature should be 1250° C. so that it is in accordance with thetemperature of the first release peak of As₂O₃ for a good fining effect.It would be very expensive and disadvantageous for the high homogeneityand reproducibility provided so far to change the melting process sothat it can be adjusted to the earlier release of oxygen in the finingprocess using CeO₂. Only in this way a fining effect can be obtainedwhich is as good as the one which is achieved by the use of the finingagent As₂O₃. However this possibility does not result in an economicalternative.

Glass ceramics comprising one or more crystal phases selected from thegroup consisting of high quartz, mixed high quartz crystal, keatite,mixed keatite crystal and β-eucryptite are preferable in the glassceramics according to the present invention.

Preferably the glass ceramics according to the present invention onlycontain small amounts of the residual gases CO₂, SO₂, O₂, argon or N₂ orcombinations thereof. The content of the residual gas, for example inZERODUR®, which has been produced by the use of toxic As₂O₃ or by theuse of fining combinations according to the present invention, such asSnO₂/SO₂ or SnO₂/Sb₂O₃, is listed in table II. From the values in tableII it becomes clear that the content of residual gas in the glassceramic with the fining combinations according to the present inventionis the same or even significantly lower than the respective contentresulting from the use of the fining agent As₂O₃.

TABLE II Content of Residual Gas in Zerodur ® for Different FiningAgents Ar/ppm N₂/ppm quali- quali- Glass CO₂/ppm SO₂/ppm O₂/ppm tativetative Production- 9.3 ± 0.9 0.8 ± 0.2 204 ± 19 <0.1 1.7 ZERODUR ®-As₂O₃ Laboratory- 10.2 ± 1.5  <0.5 228 ± 39 0.3 1 ZERODUR ®- As₂O₃Laboratory- 4.5 ± 1.5 1.2 ± 0.2 225 ± 20 <0.1 0.6 ZERODUR ®- SnO₂/SO₂Laboratory- 7.6 ± 0.9 <0.5 204 ± 20 0.3 0.3 ZERODUR ®- SnO₂/Sb₂O₃

Preferably the glass ceramic according to the present invention is usedin lithography, astronomy or as a precision element. In lithography theglass ceramic is preferably used in LCD lithography and inmicrolithography. Further preferably, the glass ceramic according to thepresent invention is used in EUV lithography.

The use of a combination of SnO₂ together with a further fining agentfor fining of glasses, which are suitable for the production of glassceramics, is also an aspect of this invention. These glass ceramics arethose described herein. A method of producing a glass ceramic accordingto this invention with the process steps described in this descriptionis also an aspect of this invention.

Mirror supports used in astronomy preferably comprise a glass ceramicaccording to the present invention. In addition mirror supports for LCDlithography and EUV lithography also preferably comprise a glass ceramicaccording to the present invention. Preferably, the mirror support is alight-weight mirror support. Prisms used in LCD lithography preferablycomprise a glass ceramic according to the present invention. Thesemirror supports and prisms are also an important aspect of the presentinvention.

While the invention has been illustrated and described as embodied intransparent LAS glass ceramics produced with alternative environmentallyacceptable fining agents and uses thereof, it is not intended to belimited to the details shown, since various modifications and changesmay be made without departing in any way from the spirit of the presentinvention.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can, by applying current knowledge,readily adapt it for various applications without omitting featuresthat, from the standpoint of prior art, fairly constitute essentialcharacteristics of the generic or specific aspects of this invention.What is claimed is new and is set forth in the following appendedclaims.

We claim:
 1. A transparent glass ceramic having low thermal expansionand a composition, in % by weight, based on oxide content, of: SiO₂  35to 70 Al₂O₃  17 to 35 Li₂O   2 to 6 TiO₂   0 to 6 ZrO₂   0 to 6 TiO₂ +ZrO₂ 0.5 to 9 ZnO 0.5 to 5,

wherein the glass ceramic is produced by fining with SnO₂ and at leastone further fining agent selected from the group consisting of Sb₂O₃,SO₄ ²⁻, Br⁻ and Cl⁻; wherein a molar ratio of the SnO₂ to the at leastone further fining agent is from 1:2 to 2:1, wherein the transparentglass ceramic has a thermal expansion coefficient in a range of−0.10·10⁻⁶/K to +0.10·10⁻⁶/K in a temperature range of 0° C. to 50° C.,and wherein the glass ceramic contains one or more crystal phasesselected from the group consisting of high quartz, mixed high quartzcrystal, keatite, mixed keatite crystal and β-eucryptite.
 2. The glassceramic according to claim 1, further comprising at least one additionalcomponent formed from a fining ingredient selected from the groupconsisting of Sb₂O₃, SO₄ ²⁻ and Cl⁻.
 3. The glass ceramic according toclaim 2, wherein said at least one additional component formed from thefining ingredient is contained in an amount of 0.001 to 2% by mol each,based on a total amount of the glass ceramic.
 4. The glass ceramicaccording to claim 1, further comprising a component formed from saidSnO₂ in an amount of 0.001 to 2% by mol.
 5. The glass ceramic accordingto claim 1, wherein said thermal expansion coefficient is from−0.05·10⁻⁶/K. to +0.05·10⁻⁶/K in said temperature range of 0° C. to 50°C.
 6. The glass ceramic according to claim 1, additionally comprisingone or more of the following components, in % by weight, based on oxidecontent: B₂O₃ 0 to 6 Na₂O 0 to 2 K₂O 0 to 2 MgO 0 to 5 P₂O₃ 0 to 17 CaO0 to 4 BaO 0 to 5 SrO 0 to
 5.


7. The glass ceramic according to claim 1, which is free of As₂O₃, CeO₂and/or PbO.
 8. An element for lithography, in particular for LCDlithography and microlithography, in astronomy or as a precisionelement, said element comprising a glass ceramic according to claim 1.9. A method of fining glass to be used for producing a transparent glassceramic, wherein said transparent glass ceramic has a low thermalexpansion and a composition, in % by weight, based on oxide content of:SiO₂  35 to 70 Al₂O₃  17 to 35 Li₂O   2 to 6 TiO₂   0 to 6 ZrO₂   0 to 6TiO₂ + ZrO₂ 0.5 to 9 ZnO 0.5 to 5,

wherein the glass ceramic is produced by fining with SnO₂ and at leastone further fining agent selected from the group consisting of Sb₂O₃,SO₄ ²⁻ and Cl⁻; wherein a molar ratio of the SnO₂ to the at least onefurther fining agent is from 1:2 to 2:1, wherein the transparent glassceramic has a thermal expansion coefficient in a range of −0.10·10⁻⁶/Kto +0.10·10⁻⁶/K in a temperature range of 0° C. to 50° C., and whereinthe glass ceramic contains one or more crystal phases selected from thegroup consisting of high quartz, mixed high quartz crystal, keatite,mixed keatite crystal and β-eucryptite, and wherein said methodcomprises fining with SnO₂ and at least one further fining agentselected from the group consisting of Sb₂O₃, SO₄ ²⁻ and Cl⁻ in a molarratio of 1:2 to 2:1.
 10. A mirror support for astronomy, said mirrorsupport comprising a glass ceramic according to claim
 1. 11. The mirrorsupport according to claim 10, which is light-weight.
 12. A prism forLCD lithography, said prism comprising a glass ceramic according toclaim
 1. 13. A mirror support for LCD lithography, said mirror supportcomprising a glass ceramic according to claim
 1. 14. A method ofproducing a glass ceramic according to claim 1, said method comprisingthe step of fining a glass melt with a combination of fining agentscomprising SnO₂ and at least one further fining agent selected from thegroup consisting of Sb₂O₃, SO₄ ²⁻, Br⁻ and Cl⁻.
 15. The glass ceramicaccording to claim 1, which is free of vanadium oxides.
 16. The glassceramic according to claim 1, containing less residual gas than a glassceramic of the same composition, except that the SnO₂ and the at leastone further refining agent are replaced by As₂O₃.